1. Field of the Invention
The present invention relates to devices which serve to modify the frequency of a light wave. The invention is also concerned with modulators which make use of a device of this type for modulating a light wave employed as a carrier wave.
2. Description of the Prior Art
Electrooptical modulators constructed in integrated optics are already known which permit amplitude modulation of a light signal by means of a modulating electric signal. In a modulator of this type as shown diagrammatically in FIG. 1, the light signal to be modulated arrives through a fiber 101 on an optical circuit which is integrated on the surface of a substrate 102. This optical circuit includes an interferometer of the Mach-Zehnder type having two arms 103 and 104 in which the light signal is shared. Two electrodes 105 and 106 extend over the surface of the substrate along one of the arms of the interferometer. The modulating electric signal V is applied to the supply terminals of said electrodes. The substrate being formed of electrooptical material such as lithium niobate, for example, the refractive index of the arm surrounded by the electrodes varies by Pockels effect. Thus at the point of junction of the two arms, the signal which has passed through the arm subjected to the action of the electrodes is phase-shifted with respect to the signal which has passed through the other arm and the intensity of the signal resulting from the sum of these two signals depends on the value of said phase shift. There is thus obtained an amplitude modulation of the optical intensity of the signal at the output of the integrated circuit. This signal then passes to an output optical fiber 107 for subsequent use in the desired manner.
It is also known to make use of optical amplitude modulators as shown in FIG. 2, in which the optical signal also arrives through an input fiber 201 on an integrated circuit at the surface of an electro-optical substrate 202. In these modulators, the optical signal is again distributed in two waveguides 204 and 203 but these latter are more closely spaced over a length L, at a distance equal to a few wavelengths. An optical coupling is accordingly obtained over this distance between the waveguides. These guides are then spaced farther apart so as to be no longer coupled and they excite two output fibers 207 and 217. Two electrodes 205 and 206 extend respectively along the two arms 203 and 204 and cover these latter. This makes it possible to modify the optical indices of these waveguides by electrooptical effect when a voltage V is applied to the terminals of the connections which supply the electrodes. In an assembly of this type, the refractive index of one of the waveguides increases whilst the index of the other waveguide decreases and vice versa as a function of the polarity of the applied voltage. In the case of this device, known as a "Y coupler", one defines a characteristic length of coupling between the two waveguides, designated as L.sub.c, which is shorter as the coupling is stronger. If the length L of the coupler is such that L=L.sub.c.sbsb..sqroot.2, there is obtained on the output fibers 207 and 217 a light intensity which varies approximately sinusoidally as a function of the voltage V, as shown in the graph of FIG. 3. If the voltage V is then caused to vary so as to remain in the central, substantially rectilinear portion of this graph, the light applied by the input fiber 201 can therefore be modulated in amplitude. The output signals on the output fibers 207 and 217 are complementary to each other.
It should be pointed out that these devices permit only amplitude modulation without modifying the frequency of the light signals. There are obtained in this case symmetrical side-bands due to the modulation, which in no way corresponds to a frequency translation.
In 1987 Ultrasonics Symposium published in the IEEE review under the reference 0090-5607/87/0000-0471, Yasumitsu Miyazaki and Nobuo Goto proposed an acoustooptical device for selectively switching optical signals as a function of the frequency. This device makes it possible to obtain a certain optical translation effect which produces the equivalent of a BLU modulation. This device, however, in which frequency translation is obtained by modulation of the refractive index under the action of acoustic waves, is limited in frequency and its construction involves a technology which is difficult to apply in practice.